1. Field of the Invention
This invention generally relates to measurement instruments, and more specifically to an adjustable gage for measuring the inside diameter of a brake drum of a vehicle while a wheel is mounted on the vehicle.
2. Description of the Prior Art
Brake drums on motor vehicles have an internal cylindrical surface arranged to be engaged by brake shoes or pads during braking. Because braking action relies on friction the brake shoes or pads as well as the cylindrical surface of the brake drums slowly wear with use and need to be adjusted or replaced in order to provide the necessary tolerances to provide reliable braking action. However, while the condition of the brake shoes or pads can frequently be established by visual inspection, it is more difficult to establish whether the brake drums fall within required specifications without utilizing a measuring instrument, such as a brake drum gage. One such brake drum gage is disclosed in U.S. Pat. No. 5,465,501 assigned to the assignee of the present application.
The problem addressed by the aforementioned U.S. Pat. No. 5,464,501 is the need to avoid inaccurate measurements resulting from improper positioning of the brake drum gage within the brake drum. The disclosed gage is provided with a plurality of pairs of adjustable depth stops that are movable along the jaws or arms of the gage which support the pointed anvils that contact the internal or external cylindrical surface to be measured. By adjusting selected depth stops to either retracted or extended positions, one pair of depth stops can be arranged to engage a rim or abutment surface of a cylindrical member, such as a brake drum, thereby ensuring that the measurement anvils will contact diametrically opposite sides of the cylindrical surface to be measured along a plane which is substantially normal to the axis of the cylindrical surface. This avoids oblique misalignments of the gage and ensures more precise measurements. However, the brake drum with adjustable depth stops disclosed in this patents is essentially a two-dimensional brake drum gage in which the jaws or arms are planar members essentially arranged in a common plane of the bar on which they are mounted. While the gage is suitable for providing very accurate measurements of a drum when the wheel is removed from the vehicle, and there are no encumbrances in the way for making the measurements on the brake drum, a gage that is two dimensional is not practical for making brake drum measurements while the wheel is mounted on the vehicle due to the presence of undercarriage and other components or parts that block free access to the cylindrical surface of the brake drum. The major obstacle or encumbrance that typically prevents such access is the wheel axle and axle tube which generally has a large diameter and extends transversely across the vehicle between opposing wheels. It is difficult, in the presence of such an axle tube, as well as other undercarriage components or parts, to position a two dimensional brake drum gage which properly aligns the measuring anvils at diametrically opposite points of the brake drum cylindrical surface, as discussed in the aforementioned patent, and the same time orients the brake drum gage in a position that makes it possible for the mechanic to read the dimensional markings or indications on the gage.